Europe's fifth-largest electricity generator, GDF Suez, is understood to have made a tentative takeover offer for the FTSE 100-listed International Power in a move likely to raise political concerns about the sale of essential British infrastructure.

GDF Suez is 35% owned by the French government. With a market value of more than £55bn, it dwarfs International Power, which owns half a dozen power stations in Britain and is worth just under £5bn. The two companies are believed to have been in talks since before Christmas; last week the French government department that deals with state shareholdings gave its approval for a bid.

GDF Suez, which also has operations in Latin America, Asia and the Middle East, has appointed the banks Rothschild, BNP Paribas and Goldman Sachs to thrash out the details of a cash and shares bid, according to weekend press reports.

While GDF Suez's share price has performed poorly on the French CAC40 index over the past year, the company does have a rumoured €12bn (£10.6bn) in cash at its disposal. Some of its shareholders have been agitating for it to use those resources to expand more aggressively.

News that a tentative approach has been made will increase pressure on the board of International Power to make an announcement to the London Stock Exchange, and a clarifying statement could come as early as tomorrow morning. There has been speculation about the future of International Power for weeks, and on Friday shares in the company closed up almost 5%.

International Power was spun out of npower a decade ago and has operations across the US, Europe, the Middle East and Asia. In the UK, it owns power stations at Derby, Rugeley in Staffordshire, Deeside in north Wales, St Austell in Cornwall and Saltend near Hull, and also has exposure to renewable energy through its First Hydro joint venture with Japan's Mitsui & Co. It generates enough electricity to power 4m homes.

Neither company would comment but any deal would put yet another part of the nation's infrastructure in foreign hands at a time when British companies still struggle to break into continental European markets and when politicians are becoming increasingly concerned about the security of gas and electricity supplies.

Lord Mandelson, the business secretary, recently warned that foreign ownership of British companies could be a "disadvantage" to the country over a long period and warned that he was "keeping a weather eye on this area".

Only two of the UK's household energy suppliers – Centrica and Scottish and Southern Energy – remain in British ownership, just over a decade after the market was opened up to full competition. International Power's former parent npower is now part of Germany's RWE.

Thames Water is owned by the Australian bank Macquarie, having previously belonged to RWE; the nuclear power company British Energy is part of the French group EDF; the airports operator BAA is owned by the Spanish infrastructure group Ferrovial; and Scottish Power is part of Spain's Iberdrola.

The debate about the UK's energy security and the role of British companies has spilled into the march towards renewable energy. The government has already tried to kickstart a green energy revolution, announcing projects that will see billions invested in offshore wind power, as well as turning to nuclear power to help reduce greenhouse gas emissions. But the first batch of contracts to create the next generation of offshore wind farms went mainly to foreign bidders.

Nine wind power consortiums have signed agreements with the Crown Estate, which has responsibility for renewable power in UK waters, to take their proposals through the planning stage. Only five of these include British companies.

First Hydro operates the pumped ­storage hydro-electric plants at Ffestiniog and Dinorwig in Snowdonia, Wales.